1. Field of the Invention
The present application relates to a magnetic recording medium, more particularly relates to the configuration of a magnetic recording medium for thermally assisted magnetic recording.
2. Description of the Related Art
In recent years, to increase the recording capacities of magnetic recording media, magnetic recording media have been raised in density. On the other hand, if magnetic recording media are raised in density, the problem of the magnetically recorded data ending up being erased due to the effects of the surrounding heat, that is, heat fluctuation, arises. To avoid the problem of heat fluctuation, it is necessary to raise the coercivity of the magnetic material used for the recording medium. However, if raising the coercivity of the magnetic material too much, existing magnetic heads can no longer be used for recording. A thermally assisted magnetic recording system uses a laser beam to heat the recording medium and record data so as to avoid the problem of heat fluctuation of the recording medium.
The thermally assisted magnetic recording system fires a laser beam on a recording layer of the recording medium through a protective layer and heats the recording layer up to near the Curie temperature to lower the coercivity and thereby enable use of an existing magnetic head for magnetic recording. In the thermally assisted magnetic recording system, when the recording layer falls in temperature after the laser beam stops being fired, the recording layer recovers in coercivity, so resistance to heat fluctuation can be given. That is, in the thermally assisted magnetic recording system, a laser beam is fired to change the temperature of the recording layer between the Curie temperature and device temperature and change the coercivity.
To improve the recording density in the thermally assisted magnetic recording system, it is necessary to reduce the write magnetic field by the head in the track width direction and simultaneously reduce the diameter of the heat spot of the recording layer reaching near the Curie temperature. Here, if defining the diameter of the heat spot as the half value width of the heat distribution and the diameter of the light spot as the half value width of the optical intensity distribution, in general the diameter of the heat spot is larger than the diameter of the light spot. This is due to the fact that when the recording layer is heated by light, the heat simultaneously diffuses in all directions. Further, the diameter of the heat spot fluctuates in size or shape due to the speed of the part heated by the light moving on the recording medium and the physical coefficient relating to the heat diffusion of the configuration material.
This thermally assisted magnetic recording system has up until now been used for removable optical disks for optomagnetic recording etc. The diameter of the light spot and diameter of the heat spot at this time have been about the wavelength. On the other hand, if aiming at a high density and large capacity with a recording capacity higher than the optical disk of over 1 Tb/inch2, the recording bit has to be tens of nm or less. The diameter of the heat spot required for a magnetic disk to realize a high density and large capacity is smaller than the wavelength of tens of nm or so. The diameter of the light spot is required to be further smaller in size. The structure of a recording medium for thermally assisted magnetic recording applied to such an optical disk is shown in Japanese Unexamined Patent Publication (Kokai) No. 2005-56504 (in particular FIG. 2). Further, the structure of a recording medium used for the magnetic disk is shown in Japanese Unexamined Patent Publication (Kokai) No. 2008-210447 (in particular FIG. 3). Furthermore, the structure of a recording medium for perpendicular recording considering the backing layer as well is shown in Japanese Unexamined Patent Publication (Kokai) No. 2005-317178 (in particular FIG. 4).
In this regard, in this thermally assisted magnetic recording system, there is the problem that in the optical head generating a small light spot of the wavelength or less, the more the diameter of the light spot is reduced below the wavelength, the more the amount of light fired on the recording medium is reduced. Therefore, to deal with the problem of the reduction of the amount of light, in the recording medium, it is important to (1) efficiently generate heat at the recording layer and (2) reduce the size of the heat distribution reaching the temperature required at the recording layer. Further, to efficiently generate heat at the recording layer, it may be considered to (1A) efficiently convert light to heat and (1B) increase the amount of light supplied to the recording layer.
To efficiently convert light to heat, it is known that it is recommended to use a material with a large complex refractive index for the material of the recording layer. Further, it is learned that to increase the amount of light supplied to the recording layer, it is sufficient to place an interference layer below the recording layer and place a reflection layer at the opposite side of the interference layer from the recording layer. For this interference layer, in general, a high refractive index material is used, while for the reflection layer, aluminum or gold is used. However, this method cannot be used in the thermally assisted magnetic recording system. The reason is that for causing interference, a light path difference of about ½ of the wavelength is necessary. For example, in the light sources used in a recording device, the wavelength of the blue semiconductor laser with a wavelength of 400 nm is the shortest. Therefore, the light path difference becomes 200 nm. However, the diameter of the light spot necessary for the thermally assisted magnetic recording system used for a magnetic disk has to be made about 1/10 or the wavelength or tens of nm or less. The size is wrong. For this reason, there is the problem that it is not possible to place an interference layer below the recording layer and increase the amount of light.
On the other hand, to reduce the size of the heat distribution reaching the temperature required at the recording layer, it is necessary that the heat not easily diffuse at the recording layer. However, if making heat hard to diffuse in the recording layer, time is required for cooling the recording layer. In the thermally assisted magnetic recording system, it is necessary to rapidly heat the recording medium, then rapidly cool it. Therefore, in the thermally assisted magnetic recording system, it is better to give directionality to the diffusion of heat so that the heat escapes downward without spreading in the recording track direction or surface direction.
In Japanese Unexamined Patent Publication (Kokai) No. 2008-210447 and Japanese Unexamined Patent Publication (Kokai) No. 2005-317178, reduction of the size of the heat spot of the recording layer was sought, but it was necessary to increase the amount of light. Another method for dealing with this problem of increased light is shown in Japanese Unexamined Patent Publication (Kokai) No. 2005-317178, but with this method, the heat ends up diffusing in the horizontal direction. This runs counter to the reduction of the heat spot. Further, there is the problem that it is not possible to use a structure of a recording medium for an optical disk when using a perpendicular magnetic recording system for the thermally assisted magnetic recording system.